Process for producing structural member of aluminum alloy

ABSTRACT

A powder preform of aluminum alloy powder is subjected to a heating treatment and then to a compacting and hardening process under a pressure to produce a structural member of aluminum alloy. The aluminum alloy powder used is one having a non-equilibrium phase which shows a calorific value C in a range of C≧10 J/g at a temperature-increasing rate of 20 K./min in a differential scanning calorimetry. In the heating treatment, the average temperature-rising rate R 2  from a heat-generation starting temperature Tx (K.) of the aluminum alloy powder to Tx+A (wherein A≧30 K.) is R 2  ≦60 K./min. Thus, the change of the non-equilibrium phase in the powder preform is uniformly performed. In addition, the average temperature-increasing rate R 4  from a processing temperature Tw (K.-B) in the compacting and hardening process to Tw (wherein B≧30 K., and Tw-B&gt;Tx+A) is R 4  ≧60 K./min. Thus, the oxidation of the powder preform is reliably prevented.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process for producing a structural member of aluminum alloy, and particularly, to a process for producing a structural member of aluminum alloy by subjecting a powder preform of aluminum alloy powder to a heating treatment and then to a compacting and hardening process under a pressure.

2. Description of the Prior Art

There is such a conventionally known process for producing a structural member having a fine metallographic structure using an aluminum alloy powder having a non-equilibrium phase (for example, see Japanese Patent Application Laid-open No. 279767/93).

In the heating treatment in the known process, the rapid increase in temperature of the powder preform is conducted at an average temperature-increasing rate R equal to or higher than 333 K./min from room temperature to a forging temperature.

The reason why such a rapid increase in temperature is conducted is that the thermal hysteresis of the powder preform is decreased and hydrogen is rapidly released from the powder preform, so that the powder preform is veiled in hydrogen and thus prevented from being oxidized.

However, when an aluminum alloy powder having a non-equilibrium phase showing a calorific value C equal to or higher than 10 J/g at a temperature-increasing rate of 20 K./min in a differential scanning calorimetry is used for the purpose of further refining the metallographic structure of the structural member, if the rapid increase in temperature equivalent to that in the known process is conducted, a problem arises that the phase change is not uniformly conducted in the powder preform and, as a result, the produced structural member has an non-uniform metallographic structure and hence, has lower mechanical characteristics.

To solve this problem, it is necessary to lower the average temperature-increasing rate during the phase change down to a value lower than that in the known process. On the other hand, it is necessary to rapidly generate the releasing of hydrogen after the phase change and hence, it is desirable to increase the average temperature-increasing rate to correspond to this.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a process of the above-described type for producing a structural member using an aluminum alloy powder specified as described above, wherein a structural member having excellent mechanical characteristics can be produced by specifying the heating conditions.

To achieve the above object, according to the present invention, there is provided a process for producing a structural member of aluminum alloy by subjecting a powder preform of aluminum alloy powder to a heating treatment and then to a compacting and hardening process under a pressure, wherein the aluminum alloy powder used is an aluminum alloy powder having a non-equilibrium phase which shows a calorific value C≧10 J/g at a temperature-increasing rate of 20 K./min in a differential scanning calorimetry, and in the heating treatment, an average temperature-increasing rate R₂ from Tx to Tx+A (wherein Tx (K.) represents a heat-generation starting temperature of the aluminum alloy powder, and A≧30 K.) is R₂ ≦60 K./min, and the average temperature-increasing rate R₄ from Tw-B to Tw (wherein Tw (K.) represents a temperature in the compacting and hardening process, and B≧30 K. and Tw-B>Tx+A) is R₄ ≧60 K./min.

The temperature range from Tx to Tx+A is a temperature range in which a non-equilibrium phase is changed. If the average temperature-increasing rate R₂ in this temperature range is set in the above-described range, the change of the non-equilibrium phase is uniformly performed, resulting in an uniformized metallographic structure of the produced structural member. It is desirable that the lower limit value for the average temperature-increasing rate R₂ is 20 K./min for inhibiting the coalescence of the metallographic structure of the structural member.

On the other hand, if the average temperature-increasing rate after the phase change is set in the above-described range, hydrogen can be rapidly released from the powder preform to reliably avoid oxidation of the powder preform. It is desirable that the upper limit value for the average temperature-increasing rate R₄ is 120 K./min for the reason that the non-uniformization of the temperature within the powder preform is prevented.

The above and other objects, features and advantages of the invention will become apparent from the following detailed description of a preferred embodiment taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing results of a differential scanning calorimetry for an aluminum alloy powder;

FIG. 2 is a graph showing one example of the relationship between the heating time and the heating temperature; and

FIG. 3 is a graph showing another example of the relationship between the heating time and the heating temperature.

DESCRIPTION OF THE PREFERRED EMBODIMENT

A molten metal having a composition of Al₉₁ Fe₆ Ti₁ Si₂ (the unit of each of the numerical values is by atom %) was prepared, and using this molten metal, an aluminum allow powder was produced by utilizing an air atomizing process. Then, the aluminum alloy powder was subjected to a classifying treatment to provide an aluminum alloy powder having a particle size of at most 45 μm.

The aluminum alloy powder was subjected to a differential scanning calorimetry (DSC). The result showed that the aluminum alloy had a non-equilibrium phase (a super-saturated solid solution) as shown in FIG. 1, which exhibited a calorific value C of 19.56 J/g at a temperature-increasing rate of 20 K./min and a heat-generation starting temperature Tx of 687.6° K. (414.6° C.).

EXAMPLE 1

Using the aluminum alloy powder, a plurality of powder preforms were formed. Then, these powder preforms were subjected to a heating treatment with an average temperature-increasing rate being varied in accordance with temperature ranges and then the powder preforms were subjected to a powder forging (compacting and hardening process) to produce a plurality of structural members.

The forming pressure for the powder preform was 600 MPa, and the powder preform has a diameter of 78 mm and a height of 20 mm. In the powder forging, the forging temperature (processing temperature) Tw was 823 K., and the forging pressure was 800 MPa. Further, the resultant structural member had a diameter of 80 mm and a height of 17 mm.

In the heating treatment, as shown in FIG. 2, the average temperature-increasing rate R₁ from room temperature RT to the heat-generation starting temperature Tx was controlled to 80 K./min; the average temperature-increasing rate R₂ from Tx to Tx+A (wherein A=30 K.) was controlled so that it was varied in a range of 40 K./min≦R₂ ≦80 K./min; the average temperature rising rate R₃ from Tx+A to Tw-B (wherein B=30 K.) was controlled to 80 K./min, and the average temperature rising rate R₄ from Tw-B to Tw was controlled so that it was varied in a range of 40 K./min≦R₄ ≦80 K./min.

Test pieces were fabricated from the structural members and subjected to a tensile test (at room temperature) and Charpy impact test to examine the relationship between the average temperature-rising rates R₁, R₂, R₃ and R₄ and the tensile strength, the elongation, as well as the Charpy impact value, thereby providing the results shown in Table 1.

                                      TABLE 1                                      __________________________________________________________________________     Average temperature-increasing rate                                            (K/min); A = 30 K, B < 30 K        Charpy                                                 R.sub.2                                                                            R.sub.3                                                                             R.sub.4                                                                             Tensile   impact                                      Test R.sub.1                                                                              (Tx to                                                                             (TX + A                                                                             (Tw - B)                                                                            strength                                                                           Elongation                                                                           value                                       piece No.                                                                           (RT to TX)                                                                           Tx + A)                                                                            to Tw - B                                                                           to Tw)                                                                              (MPa)                                                                              (%)   (J/cm.sup.2)                                                                       Estimation                              __________________________________________________________________________     1    80    80  80   75   512 2.1   9   x                                       2    80    70  80   75   518 2.4   10  x                                       3    80    60  80   75   580 6.0   18                             4    80    50  80   75   576 5.9   17                             5    80    40  80   75   581 6.0   19                             6    80    50  80   80   589 6.1   18                             7    80    50  80   70   580 6.2   19                             8    80    50  80   60   572 6.0   20                             9    80    50  80   50   481 1.0   7   x                                       10   80    50  80   40   476 0.8   7   x                                       __________________________________________________________________________

As is apparent from Table 1, if the average temperature-increasing rate R₂ is set in a range of R₂ ≦60 K./min and the average temperature-rising rate R₄ is set in a range of R₄ ≧60 K./min at A=30 K. and B=30 K., the mechanical characteristics can be largely enhanced as with the test pieces Nos.3 to 8.

The reason why such an effect is obtained is believed to be as follows: The temperature range from the Tx to Tx+A is a temperature range in which the non-equilibrium phase is changed. If the average temperature-increasing rate R₂ in this temperature range is set in the above-described range, the change of non-equilibrium phase in the powder preform is performed uniformly and hence, the metallographic structure of the structural member is uniformized. If the average temperature-increasing rate R₄ after the phase change is set in the above-described range, hydrogen can be rapidly released from the powder preform and thus, the oxidation of the powder preform can be reliably avoided.

EXAMPLE 2

Using the above-described aluminum alloy powder, a plurality of powder preforms were formed. Then, these powder preforms were subjected to heating treatment with the average temperature-increasing rate being varied in accordance with the temperature ranges, and then the powder performs were subjected to a powder forging to produce a plurality of structural members.

The forming pressure for and the size of the powder preforms, the forging temperature Tw, the forging pressure in the powder forging, and the size of the structural members were the same as those in Example 1.

In the heating treatment, as shown in FIG. 2, the average temperature-increasing rate R₁ from RT to Tx was controlled so that it was varied in a range 30 K.≦R₁ ≦100 K./min; the average temperature-increasing rate R₂ from Tx to Tx+A (wherein A=30 K.) was controlled to 50 K./min; the average temperature-increasing rate R₃ from Tx+A to Tw-B (wherein B=30 K.) was controlled so that it was varied in a range of 30 K./min≦R₃ ≦100 K./min; and the average temperature-increasing rate R₄ from Tw-B to Tw was controlled to 80 K./min.

Test pieces were fabricated from the structural members and subjected to a tensile test (at room temperature) and Charpy impact test to determine the relationship between the average temperature-increasing rates R₁ and R₃ and the tensile strength, the elongation as well as the Charpy impact value, thereby providing results shown in Table 2.

                                      TABLE 2                                      __________________________________________________________________________            Average temperature-increasing                                                 rate                                                                           (K/min); A = 30, B = 30 K                                                             R.sub.3 (Tx + A to                                                                    Tensile       Charpy impact                               Test piece No.                                                                        R.sub.1 (RT to TX)                                                                    Tx - B)                                                                               strength (MPa)                                                                        Elongation (%)                                                                        value (J/cm.sup.2)                          __________________________________________________________________________     1      100    80     579    6.0    18                                          2      70     80     583    5.8    17                                          3      50     80     580    6.1    18                                          4      30     80     581    5.9    18                                          5      80     100    589    5.9    17                                          6      80     70     580    6.1    19                                          7      80     50     571    6.4    20                                          8      80     30     561    7.0    24                                          9      100    100    594    5.7    16                                          10     30     30     560    7.0    25                                          __________________________________________________________________________

As is apparent from Tables 1 and 2, it can be seen that if the average temperature-increasing rate R₂ is set in a range of R₂ ≦60 K./min (50 K./min in Table 2) and the average temperature-increasing rate R₄ is set in a range of R₄ ≧60 K./min (50 K./min in Table 2) at A=30 K. and B=30 K., the mechanical characteristics of the test pieces Nos. 1 to 10 in Table 2 are excellent even if the average temperature-increasing rates R₁ and R₃ are varied substantially and therefore, the average temperature-increasing rates R₁ and R₃ have very little influence on the mechanical characteristics of the structural members. However, if the average temperature-increasing rate R₃ is greatly reduced, there is a tendency that the strength of the test pieces is slightly reduced as with the test pieces Nos. 8 and 10 in Table 2, whereas the elongation is enhanced.

EXAMPLE 3

Using the above-described aluminum alloy powder, a plurality of powder preforms were formed. Then, these powder preforms were subjected to a heating treatment with the average temperature-increasing rate being varied in accordance with the temperature ranges, and then the powder preforms were subjected to a powder forging to produce a plurality of structural members.

The forming pressure for and the size of the powder preforms, the forging temperature Tw, the forging pressure in the powder forging, and the size of the structural members were the same as those in Examples 1 and 2.

In the heating treatment, as shown in FIG. 2, the average temperature-increasing rate R₁ from RT to Tx was controlled to 100 K./min; the average temperature-increasing rate R₂ from Tx to Tx+A (wherein A was varied from 10 K. to 50 K.) was controlled to 50 K./min; the average temperature-increasing rate R₃ from Tx+A to Tw-B (wherein B was varied between 10 K. and 50 K.) was controlled to either 50 K./min or 80 K./min; and the average temperature-increasing rate R₄ from Tw-B to Tw was controlled to 100 K./min.

Test pieces were fabricated from the structural members and subjected to a tensile test (at room temperature) and Charpy impact test to determine the relationship between the average temperature-increasing rate R₃, Tx+A as well as Tw-B and the tensile strength, the elongation as well as the Charpy impact value, thereby providing results shown in Table 3.

                                      TABLE 3                                      __________________________________________________________________________                      Average                                                                        temperature-    Charpy                                                         increasing                                                                           Tensile   impact                                        Test piece       rate R.sub.3                                                                         strength                                                                           Elongation                                                                           value                                         No.  Tx + A (K)                                                                           Tw - B (K)                                                                           (K/min)                                                                              (MPa)                                                                              (%)   (J/cm.sup.2)                                                                       Estimation                                __________________________________________________________________________     1    Tx + 10                                                                              Tw - 30                                                                              80    510 2.1   10  x                                         2    Tx + 20                                                                              Tw - 30                                                                              80    511 2.3   11  x                                         3    Tx + 30                                                                              Tw - 30                                                                              80    581 6.1   18  0                                         4    Tx + 40                                                                              Tw - 30                                                                              80    579 6.3   19  0                                         5    Tx + 50                                                                              Tw - 30                                                                              80    578 6.2   18  0                                         6    Tx + 30                                                                              Tw - 10                                                                              50    471 1.2   6   x                                         7    Tx + 30                                                                              Tw - 20                                                                              50    474 1.0   7   x                                         8    Tx + 30                                                                              Tw - 30                                                                              50    583 6.0   18                               9    Tx + 30                                                                              Tw - 40                                                                              50    585 5.8   16                               10   Tx + 30                                                                              Tw - 50                                                                              50    589 5.9   16                               __________________________________________________________________________

As is apparent from Table 3, if A in one transition point Tx+A is set at a value≧30 K., and B in the other transition point Tw-B is set at a value≧30 K. under conditions of an average temperatures increasing rate R₂ ≦60 K./min (i.e. 50 K./min.) and an average temperature-increasing rate R₄ ≦60 K./min (i.e. 100 K./min.), the mechanical characteristics of the test pieces can largely be enhanced as with the test pieces Nos.3 to 5 and 8 to 10.

EXAMPLE 4

Molten metals having various aluminum alloy compositions were prepared, and using these molten metals, aluminum allow powders were produced by utilizing an air atomizing process. Then, the aluminum alloy powders were subjected to a classifying treatment to provide aluminum alloy powders having a particle size of at most 45 μm.

Using the aluminum alloy powders, a plurality of powder preforms were formed. Then, these powder preforms were subjected to a heating treatment, and then to a powder forging to produce a plurality of structural members.

The forming pressure for and the size of the powder preforms, the forging temperature Tw, the forging pressure in the powder forging, and the size of the structural members were the same as those in Examples 1, 2 and 3.

In the heating treatment, as shown in FIG. 3, two heating patterns P₁ and P₂ were employed. The heating patterns P₁ corresponds to an example of the present invention in which the average temperature-increasing rate R₁ from RT to Tx was controlled to 80 K./min; the average temperature-increasing rate R₂ from Tx to Tx+A (wherein A=30 K.) was controlled to 50 K./min; the average temperature-increasing rate R₃ from Tx+A to Tw-B (wherein B=30 K.) was controlled to 80 K./min; and the average temperature-increasing rate R₄ from Tw-B to Tw was controlled to 100 K./min. The other heating pattern P₂ corresponds to a comparative example in which the average temperature-increasing rate R₅ from RT to Tw-B was controlled to 120 K./min, and the average temperature-increasing rate R₆ from Tw-B to Tw was controlled to 100 K./min.

Test pieces were fabricated from the structural members and then subjected to a tensile test (at room temperature) and Charpy impact test.

Table 4 shows the composition, the calorific value C of the non-equilibrium phase at a temperature-increasing rate of 20 K./min and the heat-generation starting temperature Tx in a differential scanning calorimetry, the applied heating pattern, the tensile strength, the elongation and the Charpy impact value for the various test pieces.

                                      TABLE 4                                      __________________________________________________________________________                      Heat-                                                                          generation          Charpy                                                Calorific                                                                           starting  Tensile   impact                                    Test piece                                                                          Composition                                                                           value C                                                                             temperature                                                                          Heating                                                                            strength                                                                           Elongation                                                                           value                                     No.  (by atom %)                                                                           (J/g)                                                                               Tx (K)                                                                               pattern                                                                            (MPa)                                                                              (%)   (J/cm.sup.2)                              __________________________________________________________________________     1    Al.sub.92 Fe.sub.5 Y.sub.3                                                            52.3 625   P.sub.1                                                                            520 12.4  35                                        1a                     P.sub.2                                                                            461 5.0   12                                        2    Al.sub.90 Fe.sub.6 Ti.sub.2 Si.sub.2                                                  25.1 693   P.sub.1                                                                            591 7.2   18                                        2a                     P.sub.2                                                                            483 3.1   9                                         3    Al.sub.91 Fe.sub.6 Zr.sub.3                                                           18.0 678   P.sub.1                                                                            595 5.1   17                                        3a                     P.sub.2                                                                            433 1.1   10                                        4    Al.sub.93 Fe.sub.4 Zr.sub.1 Si.sub.2                                                  10.2 663   P.sub.1                                                                            520 9.8   21                                        4a                     P.sub.2                                                                            448 3.4   11                                        5    Al.sub.93 Cr.sub.4 Fe.sub.2 Zr.sub.1                                                  9.1  693   P.sub.1                                                                            448 4.9   11                                        5a                     P.sub.2                                                                            444 4.8   11                                        6    Al.sub.94 Ni.sub.2 Fe.sub.1 Si.sub.3                                                  0    --    P.sub.1                                                                            415 6.8   12                                        6a                     P.sub.2                                                                            419 6.9   12                                        __________________________________________________________________________

As is apparent from Table 4, if the heating pattern P₁ is employed when the aluminum alloy powder containing the non-equilibrium phase exhibiting the calorific value C equal to or more than 10 J/g is used, the mechanical characteristics of the test pieces can be largely enhanced as with the test piece Nos. 1 to 4.

If the heating pattern P₂ is employed when such aluminum alloy powder is used, the mechanical characteristics of the test pieces are reduced as with the test piece Nos. 1a to 4a.

When the calorific value C is smaller than 10 J/g, the mechanical characteristics of the test pieces are lower as with the test piece Nos. 5, 5a, 6 and 6a, irrespective of the heating patterns P₁ and P₂.

If the composition of the aluminum alloy is considered in view of the above-described results, it is believed that the desirable aluminum alloy powder is one having a composition which comprises Fe, at least one-alloy element AE selected from rare earth elements such as Y, Ti, Si and Zr, and the balance of aluminum with the content of Fe being in a range of 4 atom %≦Fe≦6 atom %, and the content of the alloy element AE being in a range of 3 atom %≦AE≦4 atom %. The present invention is applicable to the production of a structural member for an internal combustion engine, e.g., the production of a connecting rod. 

What is claimed is:
 1. A process for producing a structural member of aluminum alloy by subjecting a powder preform of aluminum alloy powder to a heating treatment and then to a compacting and hardening process under a pressure, whereinsaid aluminum alloy powder used is an aluminum alloy powder having a non-equilibrium phase which shows a calorific value C 6≧10 J/g at a temperature-increasing rate of 20 K./min in a differential scanning calorimetry/and in said heating treatment, an average temperature-increasing rate R₂ from Tx to Tx+A (wherein Tx (K.) represents a heat-generation starting temperature of the aluminum alloy powder, and A≧30 K.) is R₂ ≦60 K./min, an average temperature-increasing rate R₄ from Tw-B to Tw (wherein Tw (K.) represents a temperature in said compacting and hardening process, and B≧30 K. and Tw B-Tx+A) is R₄ ≧60 K./min.
 2. A process for producing a structural member of aluminum alloy according to claim 1, wherein said aluminum alloy powder comprises: Fe; at least one alloy element AE selected from rare earth elements, Ti, Si and Zr; and the balance of aluminum; and wherein the content of Fe is in a range of 4 atom %≦Fe≦6 atom %, and the content of said alloy element AE is in a range of 3 atom %≦AE≦4 atom %.
 3. A process for producing a structural member of aluminum alloy by subjecting a powder preform of aluminum alloy powder to a heating treatment and then to a compacting and hardening process under a pressure, whereinsaid aluminum alloy powder has a non-equilibrium phase with a calorific value C above a predetermined amount, and said heating treatment including an average temperature-increasing rate from Tx to Tx+A (wherein Tx (K.) represents a heat-generation starting temperature of the aluminum alloy powder, and A≧30 K. that is sufficiently slow to be effective for a substantially uniform change in said non-equalitorium phase, and the average temperature-increasing rate from Tw-B to Tw (wherein Tw (K.) represents a temperature in said compacting and hardening process, and B≧30 K. and Tw-B>Tx+A) that is sufficiently fast to be effective for rapidly releasing hydrogen to inhibit oxidation.
 4. A process for producing a structural member of aluminum alloy according to claim 3, wherein said aluminum alloy powder comprises: Fe; at least one alloy element AE selected from rare earth elements, Ti, Si and Zr; and the balance of aluminum; and wherein the content of Fe is in a range of 4 atom %≦Fe≦6 atom %, and the content of said alloy element AE is in a range of 3 atom %≦AE≦4 atom %.
 5. A process for producing a structural member of aluminum alloy according to claim 3, wherein said temperature Tw is the highest temperature employed in said compacting and hardening process. 